Mating backplane for high speed, high density electrical connector

ABSTRACT

A printed circuit board includes a plurality of layers including attachment layers and routing layers; and via patterns formed in the plurality of layers, each of the via patterns including first and second signal vias forming a differential signal pair, the first and second signal vias extending through at least the attachment layers; ground vias extending through at least the attachment layers, the ground vias including ground conductors; and shadow vias located adjacent to each of the first and second signal vias, wherein the shadow vias are free of conductive material in the attachment layers. The printed circuit board may further include slot vias extending through the attachment layers and located between via patterns.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/032,284, filed Jul. 11, 2018 now U.S. Pat. No. 10,455,689, which is acontinuation of U.S. application Ser. No. 15/792,953, filed Oct. 25,2017, now U.S. Pat. No. 10,034,366, which is a continuation of U.S.application Ser. No. 14/947,166 filed Nov. 20, 2015, now U.S. Pat. No.9,807,869, which claims priority based on Provisional Application No.62/082,905, filed Nov. 21, 2014, Provisional Application No. 62/172,849,filed Jun. 9, 2015, Provisional Application No. 62/172,854, filed Jun.9, 2015 and Provisional Application No. 62/190,590, filed Jul. 9, 2015,which are hereby incorporated by reference in their entirety.

BACKGROUND

This patent application relates generally to interconnection systems,such as those including electrical connectors, used to interconnectelectronic assemblies.

Electrical connectors are used in many electronic systems. It isgenerally easier and more cost effective to manufacture a system asseparate electronic assemblies, such as printed circuit boards (“PCBs”),which may be joined together with electrical connectors. A knownarrangement for joining several printed circuit boards is to have oneprinted circuit board serve as a backplane. Other printed circuitboards, called “daughter boards” or “daughter cards,” may be connectedthrough the backplane.

A known backplane has the form of a printed circuit board onto whichmany connectors may be mounted. Conductive traces in the backplane maybe electrically connected to signal conductors in the connectors so thatsignals may be routed between the connectors. Daughter cards may alsohave connectors mounted thereon. The connectors mounted on a daughtercard may be plugged into the connectors mounted on the backplane. Inthis way, signals may be routed among the daughter cards through thebackplane. The daughter cards may plug into the backplane at a rightangle. The connectors used for these applications may therefore includea right angle bend and are often called “right angle connectors.” Otherknown connectors include, but are not limited to, orthogonal midplaneconnectors and midplaneless direct attachment orthogonal connectors.

Connectors may also be used in other configurations for interconnectingprinted circuit boards and for interconnecting other types of devices,such as cables, to printed circuit boards. Sometimes, one or moresmaller printed circuit boards may be connected to another largerprinted circuit board. In such a configuration, the larger printedcircuit board may be called a “mother board” and the printed circuitboards connected to it may be called daughter boards. Also, boards ofthe same size or similar sizes may sometimes be aligned in parallel.Connectors used in these applications are often called “stackingconnectors” or “mezzanine connectors.”

Regardless of the exact application, electrical connector designs havebeen adapted to mirror trends in the electronics industry. Electronicsystems generally have gotten smaller, faster, and functionally morecomplex. Because of these changes, the number of circuits in a givenarea of an electronic system, along with the frequencies at which thecircuits operate, have increased significantly in recent years. Currentsystems pass more data between printed circuit boards and requireelectrical connectors that are electrically capable of handling moredata at higher speeds than connectors of even a few years ago.

In a high density, high speed connector, electrical conductors may be soclose to each other that there may be electrical interference betweenadjacent signal conductors. To reduce interference, and to otherwiseprovide desirable electrical properties, shield members are often placedbetween or around adjacent signal conductors. The shields may preventsignals carried on one conductor from creating “crosstalk” on anotherconductor. The shield may also impact the impedance of each conductor,which may further affect electrical properties.

Examples of shielding can be found in U.S. Pat. Nos. 4,632,476 and4,806,107, which show connector designs in which shields are usedbetween columns of signal contacts. These patents describe connectors inwhich the shields run parallel to the signal contacts through both thedaughter board connector and the backplane connector. Cantilevered beamsare used to make electrical contact between the shield and the backplaneconnectors. U.S. Pat. Nos. 5,433,617, 5,429,521, 5,429,520, and5,433,618 show a similar arrangement, although the electrical connectionbetween the backplane and shield is made with a spring type contact.Shields with torsional beam contacts are used in the connectorsdescribed in U.S. Pat. No. 6,299,438. Further shields are shown in U.S.Publication No. 2013/0109232.

Other connectors have the shield plate within only the daughter boardconnector. Examples of such connector designs can be found in U.S. Pat.Nos. 4,846,727, 4,975,084, 5,496,183, and 5,066,236. Another connectorwith shields only within the daughter board connector is shown in U.S.Pat. Nos. 5,484,310, 7,985,097 is a further example of a shieldedconnector.

Other techniques may be used to control the performance of a connector.For example, transmitting signals differentially may reduce crosstalk.Differential signals are carried on a pair of conductive paths, called a“differential pair.” The voltage difference between the conductive pathsrepresents the signal. In general, a differential pair is designed withpreferential coupling between the conductive paths of the pair. Forexample, the two conductive paths of a differential pair may be arrangedto run closer to each other than to adjacent signal paths in theconnector. No shielding is desired between the conductive paths of thepair, but shielding may be used between differential pairs. Electricalconnectors can be designed for differential signals as well as forsingle-ended signals. Examples of differential signal electricalconnectors are shown in U.S. Pat. Nos. 6,293,827, 6,503,103, 6,776,659,7,163,421, and 7,794,278.

In an interconnection system, such connectors are attached to printedcircuit boards, one of which may serve as a backplanes for routingsignals between the electrical connectors and for providing referenceplanes to which reference conductors in the connectors may be grounded.Typically the backplane is formed as a multi-layer assembly manufacturedfrom stacks of dielectric sheets, sometimes called “prepreg”. Some orall of the dielectric sheets may have a conductive film on one or bothsurfaces. Some of the conductive films may be patterned, usinglithographic techniques, to form conductive traces that are used to makeinterconnections between circuit boards, circuits and/or circuitelements. Others of the conductive films may be left substantiallyintact and may act as ground planes or power planes that supply thereference potentials. The dielectric sheets may be formed into anintegral board structure such as by pressing the stacked dielectricsheets together under pressure.

To make electrical connections to the conductive traces or ground/powerplanes, holes may be drilled through the printed circuit board. Theseholes, or “vias”, are filled or plated with metal such that a via iselectrically connected to one or more of the conductive traces or planesthrough which it passes.

To attach connectors to the printed circuit board, contact pins orcontact “tails” from the connectors may be inserted into the vias, withor without using solder. The vias are sized to accept the contact tailsof the connector.

SUMMARY

In some embodiments, a printed circuit board comprises: a plurality oflayers including attachment layers and routing layers; signal viasextending through at least the attachment layers, the signal viasincluding signal conductors; ground vias extending through at least theattachment layers, the ground vias including ground conductors; and slotvias extending through the attachment layers, the slot vias includingslot conductors that interconnect the ground planes of two or more ofthe attachment layers.

In further embodiments, a printed circuit board comprises: a pluralityof layers including attachment layers and routing layers; signal viasextending through at least the attachment layers, the signal viasincluding signal conductors; ground vias extending through at least theattachment layers, the ground vias including ground conductors; andgroups of blind plated vias extending through the attachment layers, theblind plated vias including conductors that interconnect ground planesof two or more of the attachment layers.

In further embodiments, a printed circuit board comprises: a pluralityof layers including attachment layers and routing layers; and viapatterns formed in the plurality of layers, each of the via patternscomprising: first and second signal vias forming a differential signalpair, the first and second signal vias extending through at least theattachment layers; ground vias extending through at least the attachmentlayers, the ground vias including ground conductors; and shadow viaslocated adjacent to each of the first and second signal vias, whereinthe shadow vias are free of conductive material in the attachmentlayers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the disclosed technology, reference ismade to the accompanying drawings, which are incorporated herein byreference and in which:

FIG. 1 is an exploded view of a high speed, high density electricalconnector, a backplane and a daughter board;

FIG. 2 illustrates a portion of the electrical connector of FIG. 1 whichmates with the backplane;

FIG. 3 illustrates a pin module of the electrical connector of FIG. 2;

FIG. 4 is a partial top view of a backplane having an array ofdifferential pair connections;

FIG. 5 is a simplified cross-section of a backplane;

FIG. 6 is a partial top view of an attachment layer of the backplane, inaccordance with embodiments;

FIG. 7 is a partial top view of a routing layer of the backplane, inaccordance with embodiments;

FIG. 8 is a cross-section of the backplane taken along the line 8-8 ofFIG. 6, in accordance with embodiments;

FIG. 9 is a cross-section of a signal via, in accordance withembodiments;

FIG. 10 is a cross-section of a ground via, in accordance withembodiments;

FIG. 11 is a cross-section of a slot via, in accordance withembodiments;

FIG. 12 is a cross-section of a shadow via, in accordance withembodiments;

FIG. 13 is a partial top view of an attachment layer with contoured slotvias, in accordance with embodiments;

FIG. 14 is a top view of an attachment layer having a plurality ofshield vias between differential pairs, in accordance with embodiments;

FIG. 15 is a cross-section of a shield via, in accordance withembodiments;

FIG. 16 is a top view of a backplane with offset signal conductors;

FIG. 17 is a partial top view of an attachment layer of a backplane withslots serving as ground conductors, in accordance with embodiments;

FIG. 18 is a partial top view of an attachment layer of a backplane, inaccordance with embodiments;

FIG. 19 is a partial top view of a routing layer of the backplane ofFIG. 18, in accordance with embodiments;

FIG. 20 is a schematic top view of a routing layer of a backplane, inaccordance with embodiments;

FIG. 21 is a simplified cross section of the backplane of FIG. 20, inaccordance with embodiments;

FIG. 22 is a partial top view of a routing layer of a backplane, inaccordance with embodiments;

FIG. 23 is a partial top view of an illustrative breakout layer in abackplane, in accordance with embodiments;

FIG. 24 is a partial top view of a via pattern in an attachment layer ofa backplane, in accordance with embodiments;

FIG. 25 is a partial top view of a via pattern in a routing layer of thebackplane of FIG. 24, in accordance with embodiments;

FIG. 26 is a cross-section of the via pattern of FIG. 24, in accordancewith embodiments; and

FIG. 27 is a perspective view of the via pattern of FIG. 24, inaccordance with embodiments.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that, though substantialfocus has been placed on providing improved electrical connectors inorder to improve the performance of interconnection systems, at somevery high frequencies significant performance improvement may beachieved by inventive designs for printed circuit boards. In accordancewith some embodiments, improvements may be achieved by the incorporationof structures to alter the electrical properties of the printed circuitboard in a connector footprint. The structures shown and describedherein may be utilized in any type of printed circuit board, includingbut not limited to, backplanes, mother boards, daughter boards,orthogonally mating daughter cards that mate with or without a midplaneand daughter cards that mate to a cable.

Those structures, for example, may include conducting structures,extending vertically through the board, in attachment layers of theboard, to short together edges of ground planes, which might otherwisebe free floating as a result of forming ground clearance around signalconductors. In some embodiments, the structures may be blind vias orblind slots that extend only through a portion of the layers of theboard, such as the attachment layers of the board where vias have largerdiameters to receive compliant pins or other contact tails from aconnector or other component mounted on a surface of the board. In someembodiments, the structures may be vias which are plated or filled withconductive material through some or all of the layers of the printedcircuit board. In some embodiments, the vias are not plated or filledwith conductive material through some or all of the layers of theprinted circuit board, thus forming air holes in the printed circuitboard.

FIG. 1 illustrates an example of an electrical interconnection system ofthe form that may be used in an electronic system. In this example, theelectrical interconnection system includes a right angle connector andmay be used, for example, to electrically connect a daughter card to abackplane. Two mating connectors are illustrated in FIG. 1. In thisexample, a backplane connector 100 is designed to be attached to abackplane 110 and a daughter card connector 120 is designed to beattached to a daughter card 130. Daughter card connector 120 includescontact tails 132 designed to attach to daughter card 130, and backplaneconnector 100 includes contact tails 140 designed to attach to backplane110. These contact tails form one end of conductive elements that passthrough the interconnection system. When the connectors are mounted torespective circuit boards, the contact tails make electrical connectionto conductive structures within the printed circuit boards that carrysignals or are connected to a reference potential.

Each of the connectors also has a mating interface where that connectorcan mate with or be separated from the other connector. Daughter cardconnector 120 includes a mating interface 140. Backplane connector 100includes a mating interface 142. Though not fully visible in FIG. 1,mating contact portions of the conductive elements are exposed at themating interface.

Further details of the construction of the interconnection system ofFIG. 1 are provided in FIG. 2, which shows backplane connector 100partially cut away. In the embodiment of FIG. 2, a forward wall of ahousing 210 is cut away to reveal the interior portions of matinginterface 142. In the embodiment of FIG. 2, backplane connector 100 hasa modular construction. Multiple pin modules 220 are organized to forman array of conductive elements. Each of the pin modules 220 may bedesigned to mate with a module of daughter card connector 120.

In the embodiment illustrated, four rows and eight columns of pinmodules 220 are shown. With each pin module having two signalconductors, four rows 230A, 230B, 230C and 230D of pin modules createcolumns with four pairs or eight signal conductors, in total. It will beunderstood, however, that the number of signal conductors per row orcolumn is not a limitation. A greater or lesser number of rows of pinmodules 220 may be included within housing 210. Likewise, a greater orlesser number of columns of pin modules 220 may be included withinhousing 210. Alternatively or additionally, housing 210 may be regardedas a module of a backplane connector, and multiple such modules may bealigned side-to-side to extend the length of a backplane connector.

In the embodiment of FIG. 2, each of the pin modules 220 containsconductive elements which function as signal conductors. Those signalconductors are held within insulative members, which may serve as aportion of the housing 210. The insulated portions of the pin modules220 may be positioned to separate the signal conductors from otherportions of housing 210. In this configuration, other portions ofhousing 210 may be conductive or partially conductive. Lossy orconductive members may be positioned adjacent to rows 230A, 230B, 230Cand 230D of pin modules 220. In the embodiment of FIG. 2, separators240A, 240B and 240C are provided between adjacent rows of pin modules.

FIG. 3 illustrates a pin module 220 in greater detail. Each pin module220 includes a pair of conductive elements acting as signal conductors314A and 314B. Each of the signal conductors has a mating interfaceportion shaped as a pin. Opposite ends of the signal conductors havecontact tails 316A and 316B for making electrical connections to vias ina printed circuit board, such as backplane 110. In this embodiment, thecontact tails are shaped as press-fit compliant sections. Intermediateportions of the signal conductors pass through pin module 220.

Conductive elements serving as reference conductors 342A and 342B areattached at opposite exterior surfaces of pin module 220. Each of thereference conductors has contact tails 350, shaped for making electricalconnections to vias in a printed circuit board, such as backplane 110.The reference conductors also have mating contact portions.

Embodiments of a printed circuit board are described with reference toFIG. 4. A partial top view of backplane 110 showing a connectorfootprint 410 of vias for mating with the contact tails of backplaneconnector 100 is shown in FIG. 4. The backplane 110 may be implementedas a printed circuit board as described below. As shown, the connectorfootprint 410 includes an array of rows and columns of via patterns 420.Each via pattern 420 corresponds to one differential pair of signalconductors and associated reference conductors, as well as other viasnot shown in FIG. 4 but described below. As shown, each via pattern 420includes a first signal via 430 and a second signal via 432, which forma differential signal pair, and ground vias 440, 442, 444 and 446associated with each pair of signal vias 430, 432. It will be understoodthat each of the via patterns 420 matches a pattern of contact tails316A, 316B and 350 of pin module 220 shown in FIG. 3 and describedabove. Further, the array of via patterns 420 in backplane 110 matchesthe array of pin modules 220 in backplane connector 100. It will beunderstood that the parameters of connector footprint 410 may vary,including the number and arrangement of via patterns 420 and theconfiguration of each via pattern 420, provided that the connectorfootprint 410 matches the pattern of contact tails in backplaneconnector 100.

Further embodiments of a printed circuit board are described withreference to FIG. 5. A simplified cross-sectional view of a portion ofbackplane 110 in accordance with embodiments is shown in FIG. 5. Theportion shown may be representative of a signal via in a connectorfootprint. FIG. 5 shows the layered structure of backplane 110 and asignal via 550 for purposes of illustration. It will be understood thatan actual backplane 110 includes multiple, closely spaced vias inparticular patterns as described below. The backplane 110 may beimplemented as a printed circuit board.

As further shown in FIG. 5, the backplane 110 includes multiple layers.Each layer of the multiple layers of backplane 110 may include aconductive layer and a dielectric sheet, so that the backplane 110includes an alternating arrangement of conductive layers and dielectricsheets. Each conductive layer may serve as a ground plane, may bepatterned to form conductive traces, or may include a ground plane andconductive traces in different areas. The layers may be formed, duringassembly, by stacking multiple sheets of laminate with patterned copperand prepreg and then pressing them under heat to fuse all the sheets.Patterning the copper may create traces and other conductive structureswithin the printed circuit board. As a result of fusing, the layersmight not be structurally separable in a finished backplane. However,the layers may nonetheless be recognized in the fused structure based onthe position of the conductive structures.

The layers may be allocated for different functions and accordingly mayhave different structural characteristics. In some embodiments, a firstportion of the layers, those nearest a surface, may have vias that arewide enough to receive a contact tail from a component mounted to thesurface. These layers may be called “attachment layers”. A secondportion of the layers may have narrower vias, creating wider routingchannels. These layers may be called “routing layers.”

In the illustrated embodiment, the backplane 110 includes attachmentlayers 560, 562, etc. and routing layers 570, 572, etc. The attachmentlayers are located in an upper portion of the backplane 110 and therouting layers are located below the attachment layers. The attachmentlayers 560, 562, etc. and the routing layers 570, 572, etc. are adheredtogether to form a single structure in the form of a printed circuitboard. The number of attachment layers and the number of routing layersin a particular backplane may vary according to application.

As shown in FIG. 5, backplane 110 may include ground planes 540 betweenthe layers of the structure and may include signal traces 542 in orbetween the routing layers. A signal trace 544 is shown as connected tosignal via 550.

The signal via 550 includes plating 552 in the attachment layers and inone or more of the routing layers. The signal via 550 may be backdrilledin a lower region 554 of backplane 110 to remove the plating. A groundclearance 556 is provided between signal via 550 and the ground planes540.

As further shown in FIG. 5, the via 550 has a first diameter 580 in theattachment layers and a second diameter 582 in the routing layers. Thefirst diameter 580 is larger than the second diameter 582. Inparticular, the first diameter 580 is selected to accept a contact tailof the backplane connector 100, and the second diameter 582 is selectedin accordance with typical via diameters for printed circuit boards.Because the via 550 has a relatively large first diameter 580 andbecause the vias are closely spaced to match high density backplaneconnector 100, little area remains in attachment layers 560, 562, etc.for signal routing. In routing layers 570, 572, etc. which are below thevias of the attachment layers, additional area is available for signalrouting.

In some embodiments, the vias may have the same diameter in theattachment layers and in the routing layers. For example, the contactelements of the connector may attach to pads on the surface of thebackplane 110 in a surface mount configuration.

In some embodiments, the backplane 110 may include a conductive surfacelayer 590 on its top surface. The conductive surface layer 590 ispatterned to provide an antipad 592, or non-conductive area, around eachof the signal vias. The conductive surface layer 590 may be connected tosome or all of the ground vias and may provide a contact for a connectorground, such as a conductive gasket or a conductive finger.

Further embodiments of a printed circuit board are described withreference to FIGS. 6-12. A partial top view of an embodiment of anattachment layer, such as attachment layer 560, of the backplane 110 isshown in FIG. 6. In the case of multiple attachment layers, each of theattachment layers of backplane 110 may have the same configuration. FIG.6 shows two via patterns 420 of the connector footprint 410 of backplaneconnector 100. It should be appreciated that FIG. 6 is partiallyschematic in that all of the illustrated structures may not in allembodiments be seen in a visual inspection of the top of a printedcircuit board. A coating may be placed over the board that obscures someof the structure. In addition, some structures may be formed on layersbelow the surface of the board. Those layers are nonetheless shown in atop view so that the relative position of structures in the layers maybe understood. For example, signal traces and ground planes may not bothbe visible in the same view of the board, as they are on differentvertical planes within the printed circuit board. However, because therelative positioning of signal and ground structures may be important toperformance of a printed circuit board, both may be shown in what isreferred to as a top view.

In the example illustrated, each via pattern 420 of attachment layer 560includes a first signal via 610 and a second signal via 612, which forma differential signal pair. The signal vias 610 and 612 extendvertically through the attachment layers and have diameters inattachment layer 560 that are selected to accept the contact tails 140of backplane connector 100. In forming the board, a ground plane 620 ispartially removed, such as by patterning a copper layer on a laminate,to form an antipad 622, forming a ground clearance, surrounding signalvias 610 and 612, so that the dielectric sheet of attachment layer 560is exposed. The areas where the ground plane is removed may be called“non-conductive areas” or “antipads.” The antipad 622 has a size andshape to preclude shorting of ground plane 620 to signal vias 610 and612, even if there is some imprecision in forming the vias relative toground plane 620, and to establish a desired impedance of the signalpath formed by signal vias 610 and 612. The ground plane 620 is removedaround signal vias 610 and 612 and, when the signal vias form adifferential signal pair, is removed between signal vias 610 and 612. Inthe embodiment of FIG. 6, antipad 622 is rectangular in shape, and thesignal vias 610 and 612 are centrally located in antipad 622. However,it should be appreciated that the antipad 622 may have any suitableshape, including elliptical, and may have rounded corners.

Each via pattern 420 of attachment layer 560 further includes groundvias 630, 632, 634 and 636 associated with signal vias 610 and 612. Theground vias may be disposed around the signal vias. In this example,ground vias 630 and 632 may be located at one end of the via pattern 420adjacent to signal via 610, and ground vias 634 and 636 may be locatedat an opposite end of the via pattern 420 adjacent to signal via 612.The ground vias 630, 632, 634 and 636 may be located more or less inproximity to the respective corners of rectangular antipad 622. Theground vias 630, 632, 634 and 636 are dimensioned to acceptcorresponding contact tails 140 of backplane connector 100. The groundvias interconnect the ground planes of some or all of the layers of thebackplane 110. In particular, the ground vias may extend through all ofthe layers of the backplane 110 and may be plated with a conductivematerial.

Each via pattern 420 of attachment layer 560 further includes shadowvias 640, 642, 644 and 646. The shadow vias 640 and 642 are located onopposite sides of signal via 610, and shadow vias 644 and 646 arelocated on opposite sides of signal via 612. As shown in FIG. 6, theshadow vias overlap the edges of antipad 622 and are positionedrelatively closely to the respective signal vias. The shadow vias do notaccept contact tails of backplane connector 100 and, in the attachmentlayers of backplane 110, are not plated with a conductive material. Insome embodiments, for example, the shadow vias may be formed byinitially forming and plating a via. The plating on that via may then beremoved, such as by drilling, sequential lamination of the printedcircuit board, or by any other suitable technique. In some embodiments,the plating may be removed only in the attachment layers. The plating onthe vias may remain through some or all of the routing layers. Removingthe plating on the shadow vias in the attachment layers effectivelyincreases the distance between the signal vias 610 and 612 and thenearest ground structure in the attachment layers where the signal viashave a larger diameter than in the routing layer. This area of removedplating both reduces the risk of shorting, from inaccuracies inpositioning signal vias relative to ground vias, and also may provide amore uniform impedance along the signal paths formed by the signal viapairs.

In particular, the removal of plating in shadow vias 640, 642, 644 and646, such as by drilling, effectively provides air holes adjacent tosignal vias 610 and 612. The air holes may increase the impedance alongthe signal paths and thereby improve performance.

However, removing plating from a ground via to form shadow vias removesground structures between adjacent via patterns 420, which may enablecross talk between signal conductors in adjacent via patterns. Inaddition, edges of ground plane 620 adjacent the signal vias 610 and 612are not electrically tied together. As a result, the space betweenground planes near the signal vias 610 and 612 may be electricallyexcited by signals traveling along the vias. Excitation may generateresonance, which may spread throughout the interconnection system,creating cross talk and other problems. One or more conductingstructures may extend through the printed circuit board to connect thoseedges of the ground planes together, substantially reducing the chanceof resonance. In the illustrated embodiment, the ground planes may beconnected using slot vias.

The connector footprint 410 of FIG. 6 further includes slot vias 650positioned between adjacent via patterns 420. Slot vias 650 may have theform of an elongated hole that extends only partially through theprinted circuit board, such as only through the attachment layers of thebackplane 110 but does not extend through the routing layers. The slotvias 650 are located between shadow vias of adjacent via patterns 420and may have a length that is greater than the spacing between signalvias 610 and 612. In the embodiment of FIG. 6, the slot vias 650 have alength that is roughly equal to the long dimension of the antipad 622.The slot vias 650 are plated with a conductive material and interconnectthe ground planes of the attachment layers. Further, the slot vias 650may have the incidental effect of providing electrical shielding betweenthe signal vias of adjacent differential signal pairs. Due to the longdimension of the slot vias, plating the slot vias 650 with a conductivematerial is easier than the plating of circular holes of comparablewidth. In one non-limiting example, slot vias 650 have lengths of 3.175mm and widths of 0.5 mm.

A partial top view of an embodiment of a routing layer, such as routinglayer 570, of backplane 110 is shown in FIG. 7. In the case of multiplerouting layers, each of the routing layers may have the sameconfiguration, except that different electrical connections are made tothe via patterns. However, in some embodiments, some of the signal viasand/or ground vias may be backdrilled, removing conductive plating closeto a lower surface of the printed circuit board. Two via patterns 720are shown in FIG. 7. It will be understood that via patterns 720 arevertically aligned under respective via patterns 420 of the pin layers.It will further be understood that the via patterns 720 are not visiblein the backplane 110 after the layers of the backplane are pressedtogether, such that FIG. 7 may be regarded as a schematic illustrationof the routing layer.

Each via pattern 720 of routing layer 570 includes signal vias 610 and612 which extend vertically through the attachment layers and at leastone of the routing layers. However, the signal vias 610 and 612 havesmaller diameters in the routing layers than in the attachment layers.In particular, signal vias have a first diameter in the attachmentlayers and a second diameter in the routing layers, wherein the seconddiameter is smaller than the first diameter. The signal vias 610 and 612can have smaller diameters in the routing layers because they are notrequired to accept the contact tails 140 of the backplane connector 100.In the via pattern 720, a ground plane 730 is partially removed to forman antipad 732 surrounding signal vias 610 and 612. The antipad 732 ofrouting layer 570 may have the same size and shape as the antipad 622 ofattachment layer 560. However, this is not a requirement, as in someembodiments, the separation between the signal vias and the edges of theground plane may be selected at each layer to provide a desiredimpedance or to otherwise provide desired electrical properties.

Each via pattern 720 of routing layer 570 also includes ground vias 630,632, 634 and 636 which have the same locations and configurations as thecorresponding ground vias in pin layer 560. In particular, the groundvias 630 and 632 are located at one end of the via pattern 720 adjacentsignal via 610, and ground vias 634 and 636 are located at an oppositeend of the via pattern 720 adjacent to signal via 612. The ground viasin the routing layers are not required to accept contact tails of thebackplane connector, but may have the same diameters as the ground viasin the attachment layers. The ground vias 630, 632, 634 and 636 in therouting layers can be plated or filled with a conductive material. Asnoted, the ground vias typically interconnect the ground planes of allthe layers of the backplane 110.

Each via pattern 720 of routing layer 570 further includes shadow vias640, 642, 644 and 646 which extend vertically from the attachmentlayers. In the routing layers, the shadow vias are plated or filled witha conducting material. Because the signal vias 610 and 612 have smallerdiameters in the routing layer 570, the spacing between signal vias 610and 612 and the corresponding shadow vias is larger than in theattachment layer 560.

The slot vias 650 shown in FIG. 6 do not extend into the routing layers.This facilitates use of the routing layers for routing of signal tracesto respective signal vias.

A schematic cross-section of the via pattern 420 of FIGS. 6 and 7 isshown in FIG. 8. The interrelationship between the vias of via pattern420 at different depths in backplane 110 is shown in FIG. 8. The viatypes are described in greater detail below. As shown, signal via 610extends through attachment layers 560, 562, etc. and one or more ofrouting layers 570, 572, etc., and provides a signal connection to atleast one of the routing layers. Shadow vias 640 and 642 are located onopposite sides of signal via 610 and are plated with a conductivematerial in the routing layers. In the embodiment of FIG. 8, shadow vias640 and 642 are not plated with a conductive material in the attachmentlayers. The shadow vias 640 and 642 interconnect ground planes of therouting layers, but, in the embodiment of FIG. 8, do not interconnectground planes of the attachment layers. The slot vias 650 are located onopposite sides of shadow vias 640 and 642 and extend through only theattachment layers 560, 562, etc. Slot vias 650 are connected to theground planes of each of the attachment layers. As shown, the slot vias650 and the conductive portions of shadow vias 640 and 642 share theground plane of at least one routing layer 570.

A schematic cross-section of an embodiment of signal via 610 is shown inFIG. 9. Signal via 612 may have the same configuration. As shown, thesignal via 610 extends through the attachment layers and through atleast one of the routing layers of backplane 110. Such a via may beformed by drilling a hole fully through the board, plating the via andthen removing portions of the plating adjacent the lower surface of theboard.

Signal via 610 has a first region 900 having a first diameter 910 and afirst length 912 and a second region 914 having a second diameter 920and a second length 922. In general, the first region 900 extendsthrough the attachment layers and the second region 914 extends throughat least one of the routing layers. The first diameter 910 is largerthan the second diameter 920. As indicated previously, the firstdiameter 910 is selected to accept a contact tail 140 of backplaneconnector 100. The signal via 610 is plated with a conductive materialthroughout its entire length. The signal via 610 may pass through acontact pad 930 on the top layer of the backplane 110 and a contact pad932 on the layer where the signal via is connected.

In one non-limiting example, the first diameter 910 of signal via 610 is15.7 mils, the first distance 912 is 50 mils, the second diameter is 920is 11 mils and the second distance 922 is 26 mils. It will be understoodthat these dimensions are not limiting and that other dimensions may beutilized.

A schematic cross-section of an embodiment of ground via 630 is shown inFIG. 10. Ground vias 632, 634 and 636 may have the same configuration.As shown, the ground via 630 extends through the attachment layers andthrough the routing layers of backplane 110. In some embodiments, theground via 630 is formed as a through hole that extends through theentire thickness of backplane 110. In the embodiment of FIG. 10, groundvia 630 has a uniform diameter 1010 throughout its length. However,ground via 630 is not required to have a uniform diameter, provided thatthe diameter in the attachment layers of backplane 110 is sufficient toaccept a contact tail of backplane connector 100. The ground via 630includes contact pads 1020 on some or all of the attachment layers andthe routing layers. In some embodiments, the ground via 630 passesthrough multiple ground planes, indicated by contact pads 1020 on eachof the attachment layers and the routing layers.

In one non-limiting example, the ground via 630 has a diameter of 15.7mils and includes contact pads having diameters of 25.7 mils on everyground layer. It will be understood that these dimensions are notlimiting and that other dimensions may be utilized.

A schematic cross-section of an embodiment of slot via 650 is shown inFIG. 11. As shown, the slot via 650 extends through the attachmentlayers of backplane 110 but does not extend through the routing layers.Further, slot via 650 may be uniform in cross-section along its length1110. The slot via 650 is not required to accept a contact tail ofbackplane connector 100. The slot via 650 is plated with a conductivematerial throughout its entire length and may be connected to the groundplanes in each of the attachment layers. The slot via 650 functions as ashield between the signal vias of adjacent differential signalconductors and also interconnects the ground planes of the attachmentlayers. The slot via 650 may pass through and connect to ground planes,indicated by contact pads 1120 at each of the attachment layers.

In the absence of connections between the ground planes of theattachment layers, the ground planes form a “cavity” with two opposingconductive sheets that are not terminated at one end. For high frequencysignals this cavity may resonate, producing undesirable results such aspreading cross talk throughout the backplane. By interconnecting theground planes of the attachment layers, such resonance is suppressed.

The slot via 650 may be located between pairs of differential signalvias. Referring to FIG. 6, each via pattern 420 includes differentialsignal vias 610 and 612. The four signal vias of two adjacent signalpairs define a rectangle, with each pair forming one of two oppositesides of the rectangle. The signal vias 650 are located generally in themiddle region of this rectangle between the two adjacent signal pairs,thus providing signal isolation between the two signal pairs.

The slot via 650 has a cross-section that is elongated in one direction,as shown in FIG. 6. Thus, the slot via 650 may have a cross-section inthe form of an oval or a rectangle but is not limited to these shapes.As discussed below, the slot vias may have an irregular, or contoured,shape. In the example of FIG. 6, slot via 650 has an oval shape.

In one non-limiting example, slot via 650 has a length of 3.175 mm, awidth of 0.50 mm and a depth of 50 mils. It will be understood thatthese dimensions are non-limiting and that other dimensions may beutilized.

The slot via 650 enables more efficient and reliable plating of theblind slot compared to a blind circular via due to increased circulationand replenishment of plating fluid in a blind slot having a smallestdimension equal to the diameter of the circular via. The slot via 650provides electrical shielding between the two adjacent signal pairs inthe region of the backplane 110 through which the plated slot extends.In some embodiments, a ground attachment between the connector and thebackplane can be provided. The attachment can take the form of a rippledor ridged metal ground tab interferingly inserted into the slot, oranother configuration such as a row of small twisted blades extendingfrom the connector. The blades are designed to torsionally deform uponinsertion into the plated slot and thus make electrical and mechanicalconnections to the ground conductors of the backplane 110.

A schematic cross-section of an embodiment of shadow via 640 is shown inFIG. 12. Shadow vias 642, 644 and 646 may have the same configuration.As shown, shadow via 640 extends through the attachment layers andthrough the routing layers of backplane 110. The shadow via 640 includesa first region 1210 having a first length 1212 and a second region 1220having a second length 1222. The shadow via 640 may be formed as athrough hole that extends through the thickness of backplane 110.However, shadow via 640 is not limited to a through hole. In theembodiment of FIG. 12, the first region 1210 of shadow via 640, whichextends through the attachment layers of backplane 110, is not platedwith conductive material, whereas the second region 1220 of shadow via640 is plated with a conductive material. In some embodiments, shadowvia 640 is not plated in the attachment layers, such as may result fromdrilling away plating after it is deposited. The depth of the non-platedfirst region 1210 of shadow via 640 may be somewhat less than, equal toor somewhat greater than the depth of the region 910 of signal via 610which accepts the contact tail of backplane connector 100. Shadow via640 is provided with contact pads 1230 which may connect to some or allground planes in second region 1220. The shadow via 640 is not requiredto accept a contact tail of backplane connector 100.

In one non-limiting example, the shadow via has a diameter of 12 mils inthe first region 1210. In the first region 1210 the first length 1212 is43 mils, and in the second region 1220 the second length 1222 is 42mils. The contact pads 1230 may have diameters of 18 mils. It will beunderstood that these dimensions are not limiting and that otherdimensions may be utilized.

Further embodiments of a printed circuit board are described withreference to FIG. 13. A partial top view of an alternative embodiment ofan attachment layer, such as attachment layer 560, is shown in FIG. 13.In the case of multiple attachment layers, each of the attachment layersof backplane 110 may have the same configuration. FIG. 13 shows two viapatterns 1320 of the connector footprint 410 of backplane 110.

Each via pattern 1320 of attachment layer 560 includes signal vias 610and 612, which form a differential signal pair, ground vias 630, 632,634 and 636 and shadow vias 640, 642, 644 and 646. The signal vias, theground vias and the shadow vias in FIG. 13 may correspond to therespective vias shown in FIG. 6 and described above. Accordingly, theirdescriptions will not be repeated. The ground plane 620 is partiallyremoved to form an antipad 1322 surrounding signal vias 610 and 612, sothat the dielectric sheet of attachment layer 560 is exposed.

The connector footprint of FIG. 13 further includes contoured slot vias1350 positioned between adjacent via patterns 1320. Contoured slot vias1350 may extend through the attachment layers of backplane 110, but donot extend through the routing layers. The contoured slot vias 1350 arelocated between shadow vias of adjacent via patterns 1320. The contouredslot vias 1350, as shown in FIG. 13, may have the form of an elongatedslot with outward bumps 1360 and 1362 on opposite sides of the elongatedslot. The outward bumps 1360 and 1362 may extend toward respective pairsof signal vias between pairs of shadow vias. Thus, for example, outwardbump 1360 of contoured slot via 1350 extends toward signal vias 610 and612 between shadow vias 640 and 644, and outward bump 1362 extendstoward signal vias 610 and 612 between shadow vias 642 and 646. Thecontoured slot vias 1350 are plated with a conductive material andinterconnect the ground planes of the attachment layers. Further, thecontoured slot vias 1350 provide electrical shielding between the signalvias of adjacent differential signal pairs. The contoured slot vias 1350are not limited to the cross-sectional shapes shown and describedherein.

In one non-limiting example, contoured slot via 1350 has a length of3.175 mm and a width of 0.5 mm. Outward bumps 1360 and 1362 each have aheight of 0.635 mm and a width of 1.04 mm. It will be understood thatthese dimensions are not limiting and that other dimensions may beutilized.

Further embodiments of a printed circuit board are described withreference to FIGS. 14 and 15. A partial top view of a further embodimentof an attachment layer, such as attachment layer 560, of the backplane110 is shown in FIG. 14. In the case of multiple attachment layers, eachof the attachment layers of backplane 110 may have the sameconfiguration. FIG. 14 shows two via patterns 1420 of the connectorfootprint 410 of backplane connector 100.

Each via pattern 1420 of attachment layer 560 includes signal vias 610and 612, which form a differential signal pair, ground vias 630, 632,634 and 636, and shadow vias 640, 642, 644 and 646 as described above inconnection with FIG. 6. Accordingly, the descriptions of these vias willnot be repeated.

The connector footprint 410 of FIG. 14 further includes groups of blindplated vias positioned between adjacent via patterns 1420. In theembodiment of FIG. 14, blind plated vias 1430 and 1432 are locatedbetween shadow vias of adjacent via patterns 1420. In addition, blindplated vias 1440 and 1442 are located along the left side of antipad 622between shadow vias 640 and 644; and blind plated vias 1450 and 1452 arelocated along the right side of antipad 622 between shadow vias 642 and646. Thus, a group of blind-plated vias associated with each via pattern1420 includes blind plated vias 1430, 1432, 1440, 1442, 1450 and 1452.Together, the group of blind plated vias functions similarly to slotvias 650 and contoured slot vias 1350 with respect to providingshielding between adjacent via patterns 1420 and with respect tointerconnecting the ground planes of the attachment layers.

A schematic cross-section of an embodiment of blind plated via 1430 isshown in FIG. 15. Blind plated vias 1432, 1440, 1442, 1450 and 1452 mayhave the same configuration. As shown, blind plated via 1430 extendsthrough the attachment layers of backplane 110, but does not extendthrough the routing layers. As with other blind vias, such a structuremay be formed by drilling a hole through the board, plating the hole andthen drilling away the plating where the via is not desired. However,any suitable approach may be used to form a blind via.

The blind plated via 1430 is not required to accept a contact tail ofthe backplane connector 100. The blind plated via 1430 may be platedwith a conductive material throughout its entire length and may includeone or more contact pads 1520 for connection to ground planes of the pinlayers. In some embodiments, the blind plated via 1430 contacts theground plane of every pin layer in backplane 110, as represented by pads1520.

In one non-limiting example, the blind plated via 1430 has a diameter of15.7 mils and a length 1530 of 50 mils. The contact pads 1520 can havediameters of 21.7 mils. It will be understood that these dimensions arenot limiting and that other dimensions may be utilized.

Further embodiments of a printed circuit board are described withreference to FIG. 16. A partial top view of an embodiment of backplane110 showing an alternate connector footprint 1610 for mating with thecontact tails of the backplane connector is shown in FIG. 16. Theconnector footprint 1610 includes an array of columns of via patterns420. In the connector footprint 1610, alternating columns of viapatterns 420 are offset in the column direction. In particular, a firstcolumn 1620 of via patterns 420 is offset from a second column 1622 ofvia patterns in the column direction by a distance 1630 which may be onehalf of the vertical dimension of via pattern 420. Similarly, secondcolumn 1622 of via patterns 420 may be offset from a third column 1624of via patterns 420 by the distance 1630. The individual via patternsmay correspond to the via patterns shown and described herein. Inparticular, the via patterns shown and described herein may be arrangedaccording to the connector footprint 410 of FIG. 4 with non-staggeredcolumns, the connector footprint 1610 of FIG. 16 with staggered columns,or any other suitable connector footprint.

Further embodiments of a printed circuit board are described withreference to FIG. 17. An embodiment of a via pattern is shown in FIG.17. A via pattern 1720 includes signal vias 610 and 612 as describedabove. The via pattern 1720 further includes ground vias 1730 and 1732in the form of slots rather than pairs of ground vias as describedabove. The slot-shaped ground vias 1730 and 1732 may extend through theattachment layers and the routing layers of the backplane 110 and mayconnect to the ground planes of each of the attachment layers and, insome embodiments, the routing layers. Such slots may be used instead ofor in addition to slots 650 or other conductive structures similarlypositioned.

The contact tails of backplane connector 100 may be shaped to match theslot-shaped ground vias 1730 and 1732. In other embodiments, the contacttails of the backplane connector 100 are replaced by tabs that areinserted into the slot-shaped ground vias 1730 and 1732. The slot-shapedground vias 1730 and 1732 provide shielding of the differential signalvias. The use of a tab, rather than two individual contact tails,distributes current more evenly. A tab that fits into a slot providesshielding even if the connector is not pressed all the way into thebackplane. With separate contact tails, if the connector is not pressedall the way into the vias, there is a separation between the bottom ofthe shield on the connector and the top of the backplane. This does notoccur with a tab pressed into a slot. Further, slot-shaped ground viasprovides shielding even if a tab is not inserted into them.

Further embodiments of a printed circuit board are described withreference to FIGS. 18 and 19. A partial top view of an alternativeembodiment of an attachment layer, such as attachment layer 560, isshown in FIG. 18. In the case of multiple attachment layers, each of theattachment layers of backplane 110 may have the same configuration. FIG.18 shows two via patterns 1820 of the connector footprint 410 ofbackplane 110.

Each via pattern 1820 of attachment layer 560 includes signal vias 610and 612, which form a differential signal pair, ground vias 630, 632,634 and 636 and shadow vias 640, 642, 644 and 646. The signal vias, theground vias and the shadow vias 640, 642, 644 and 646 in FIG. 18 maycorrespond to the respective vias shown in FIG. 6 and described above.Accordingly, their descriptions will not be repeated. The ground plane620 is partially removed to form an antipad 1822 surrounding signal vias610 and 612, so that the dielectric sheet of attachment layer 560 isexposed around and between signal vias 610 and 612.

Each via pattern 1820 shown in FIG. 18 further includes shadow vias 1840and 1842 positioned adjacent to respective signal vias 610 and 620. Inparticular, shadow vias 1840 and 1842 may be offset in a direction of aline 1850 passing through signal vias 610 and 612. In some embodiments,shadow vias 1840 and 1842 may be located along the line 1850. Moreparticularly, shadow via 1840 may be located between signal via 610 andground vias 630 and 632, and shadow via 1842 may be located betweensignal via 612 and ground vias 634 and 636. As further shown in FIG. 18,shadow vias 1840 and 1842 may be located on the short sides of antipad1822, approximately at the centers of the short sides thereof. Shadowvias 1840 and 1842 are not dimensioned to accept contact tails ofbackplane connector 100.

The shadow vias 1840 and 1842 extend through the attachment layers ofbackplane 110 and may extend through one or more of the routing layers.In some embodiments, the shadow vias 1840 and 1842 may extend throughall of the layers of the backplane 110 and may be plated with aconductive material. In some embodiments, the shadow vias 1840 and 1842are not plated with a conductive material in the attachment layers. Inother embodiments, the shadow vias 1840 and 1842 may be plated or filledwith a conductive material in the attachment layers.

The shadow vias 1840 and 1842 are disposed parallel to and in closeproximity to respective signal vias 610 and 612. In some embodiments,shadow vias 1840 and 1842 may not be plated in the attachment layers,such as may result from drilling away plating after it is deposited.When plated with conductive material in the attachment layers, shadowvias 1840 and 1842 may provide a current return path for signal vias 610and 612. When not plated in the attachment layers, the shadow vias 1840and 1842 may provide increased clearance between the signal vias 610 and612 and the nearest ground. The shadow vias, when drilled away, maylower the dielectric constant of the material between the signal vias610 and 612 and the nearest ground, which may provide an increasedimpedance along the signal vias in the attachment layers. This increasedimpedance may provide better performance of the interconnection systemby matching the impedance in the attachment layers to the routinglayers, a connector or other portions of the interconnection system.

In one non-limiting example, shadow vias 1840 and 1842 may have the samedimensions as shadow via 640 shown in FIG. 12 and discussed above.However, the dimensions of shadow vias 1840 and 1842 are not required tobe the same as those of shadow via 640. It will be understood that thesedimensions are not limiting and that other dimensions may be utilized.

A partial top view of an embodiment of a routing layer, such as routinglayer 570, of backplane 110 is shown in FIG. 19. In the case of multiplerouting layers, each of the routing layers may have the sameconfiguration, except that different electrical connections are made tothe via patterns. In some embodiments, some of the signal vias and/orground vias and/or shadow vias may be backdrilled, removing conductiveplating close to a lower surface of the printed circuit board.

Two via patterns 1920 are shown in FIG. 19. It will be understood thatvia patterns 1920 are vertically aligned under respective via patterns1820 of the attachment layers, as shown in FIG. 18. It should beappreciated that the via patterns 1820 and the via patterns 1920 may bepositioned to create the connector footprint 410 with non-staggeredcolumns, as shown in FIG. 4, the connector footprint 1610 with staggeredcolumns, as shown in FIG. 16, or any other suitable connector footprint.It will further be understood that the via patterns 1920 are not visiblein the backplane 110 after the layers of the backplane are pressedtogether, such that FIG. 19 may be regarded as a schematic illustrationof the routing layer.

Each via pattern 1920 of routing layer 570 includes signal vias 610 and612 which extend vertically through the attachment layers and at leastone of the routing layers. As described above, the signal vias 610 and612 have smaller diameters in the routing layers than in the attachmentlayers. In each via pattern 1920, the ground plane 730 is partiallyremoved to form an antipad 1932 surrounding and between signal vias 610and 612. The antipads 1932 of routing layer 570 may have the same sizeand shape as the antipad 1822 of pin layer 560. However, this is not arequirement as discussed below.

Each via pattern 1930 of routing layer 570 also includes ground vias630, 632, 634 and 636 which have the same locations and configurationsas the corresponding ground vias in attachment layer 560. Each viapattern 1920 of routing layer 570 also includes the shadow vias 640,642, 644 and 646 which extend vertically from the attachment layers. Inthe routing layers, the shadow vias may be plated or filled with aconducting material. The slot vias 650 shown in FIG. 18 do not extendinto the routing layers and are not shown in FIG. 19.

Each via pattern 1920 of routing layer 570 further includes shadow vias1840 and 1842 as described above in connection with FIG. 18. The shadowvias 1840 and 1842 extend at least to the routing layer where the signalvias 610 and 612 are connected to signal traces. The shadow vias 1840and 1842 may extend from the routing layer where the signal vias 610 and612 are connected to signal traces to the back side of the backplane110, or may be backdrilled from the back side of backplane 110.

Further embodiments of a printed circuit board are described withreference to FIGS. 20 and 21. While not being bound by any particulartheory of operation, the inventors have recognized and appreciated thatat high frequencies, antipads around a pair of signal conductors createwhat can be electrically equivalent to a cavity, bounded by conductivestructures, around the signal conductors. The inventors have furtherrecognized and appreciated that coupling of signals between traces inthe printed circuit and within the vias can excite resonances withinthat cavity. Those resonances can reduce the performance of theinterconnection system including the printed circuit board. Accordingly,structuring the transition regions of the printed circuit board so as toreduce the excitation of resonant modes within that cavity may increaseperformance of the interconnection system.

Approaches for reducing the excitation of resonances are describedbelow. In accordance with some embodiments, configuring ground planes ina via pattern in routing layers above and below a “breakout layer” inwhich traces within the printed circuit board are connected to thesignal vias may reduce the risk of exciting resonant modes in thecavity.

In some embodiments, reduction in resonances may be achieved by groundstructures adjacent the breakout layer extending into or toward thespace between the signal vias, without entirely bridging opposing sides.Such a configuration, it is theorized, guides return current in theground conductor in a mode transition appropriate for a transition inorientation of the conductors between signal vias and traces on theprinted circuit board, which are generally oriented at right angles.

A schematic top view, including a routing layer 2010 of backplane 110,is shown in FIG. 20. The routing layer 2010 may be the routing layerthat is immediately below a breakout layer 2112 (FIG. 21) where signalvia 610 is connected to a signal trace 2140. The routing layer 2010 mayhave a ground plane 2022. Additionally, a ground layer 2024, in arouting layer above the breakout layer 2112, is shown. In this example,an antipad 2026 in ground layer 2024 is larger than the antipads 2030and 2032 in ground layer 2022, such that portions of ground layer 2022are illustrated as visible through antipad 2026. The antipad 2026 isindicated by cross-hatching in FIG. 20. It should be appreciated, aswith other top-down views, that only some of the structures of a printedcircuit board are illustrated, omitting others so as to reveal thestructures illustrated. For example, a dielectric matrix holding theconductive structures is not shown. Likewise, conductive structures ononly selected layers are shown. The antipads in ground layers 2022 and2024 are superimposed in FIG. 20 for purposes of illustration.

FIG. 20 shows two via patterns 2020 of routing layer 2010. A simplifiedcross-sectional view of a portion of backplane 110 corresponding to FIG.20 is shown in FIG. 21. FIG. 21 is a cross-sectional view of the portionaround signal via 610.

Each via pattern 2020 of routing layer 2010 includes signal vias 610 and612 which extend vertically through the attachment layers and throughone or more of the routing layers, including routing layer 2010 andbreakout layer 2112. In each via pattern 2020, the ground plane ispartially removed to form a non-conductive area, or antipad, surroundingsignal vias 610 and 612. In the embodiment illustrated, the antipad 2026is rectangular and is large enough for both signal vias 610 and 612 topass through. In the embodiment illustrated, antipad 2026 isrepresentative of an antipad for the signal vias 610 and 612 in groundplanes of the routing layers above the breakout layer 2112. As discussedabove, antipad 2026 and similar antipads in other layers of the printedcircuit board define what can be electrically equivalent to a cavity.The cavity has a long dimension parallel with a line 2040 (FIG. 20)between signal vias 610 and 612.

For simplicity of illustration, the traces connected to signal vias 610and 612 are not shown in FIG. 20. However, such traces would exist inbreakout layer 2112. A representative breakout layer is shown in FIG.22, as described below. Briefly, FIG. 22 illustrates that traces on abreakout layer connect to the signal vias and then are routed to otherportions of the printed circuit board as a pair.

The traces connected to signal vias 610 and 612 are not illustrated inFIG. 20 so as to reveal the configuration of the ground plane 2024 (FIG.21). In the example of FIGS. 20 and 21, ground plane 2022 may also haveantipads surrounding the signal vias 610 and 612, or at least openingsthrough which the vias are drilled away in routing layers below thebreakout layer. A non-conductive area, or antipad 2030, surroundingsignal via 610 and a non-conductive area, or antipad 2032, surroundingsignal via 612 are shown. The antipads 2030 and 2032 may be circular inshape to match the respective signal vias 610 and 612 and may havesufficient diameter to avoid contact between the ground plane 2022 andthe signal vias 610 and 612, taking into account production variationsin size and position. It may be noted that the ground plane 2022 remainsin an area 2028 between signal vias 610 and 612, since operation as adifferential signal pair is not required in routing layers belowbreakout layer 2112.

Each via pattern 2020 of routing layer 2010 may also include ground vias630, 632, 634 and 636, which may have the same locations andconfigurations as the corresponding ground vias in the attachment layer560, as shown in FIG. 6. Each via pattern 2020 of routing layer 2010 mayalso include shadow vias 640, 642, 644 and 646 which extend verticallyfrom the attachment layers. In the routing layers, the shadow vias maybe plated or filled with a conductive material. The slot vias 650 shownin FIG. 6 do not extend into the routing layers and are not shown inFIG. 20.

As shown in FIG. 21, the backplane 110 includes attachment layers 560,562, etc. and routing layers 570, 572, 2112 and 2010, etc. The number ofattachment layers and the number of routing layers in a particularbackplane may vary according to application. As further shown in FIG.21, backplane 110 may include ground planes 540 between layers of thestructure and may include signal traces 542 in or between the routinglayers. Signal trace 2140 in breakout layer 2112 is connected to signalvia 610.

The signal via 610 includes plating 2142 in the attachment layers and inone or more of the routing layers. A ground clearance 2144 is providedbetween signal via 610 and the ground planes 540 in the attachmentlayers and in one or more of the routing layers. The ground clearance2144 in the attachment layers corresponds to the clearance provided byantipad 622 shown in FIG. 6 and described above. The ground planes 540are removed in the region around and between signal vias 610 and 612 toform antipads on each of the attachment layers and on each of therouting layers down to and including breakout layer 2112.

As further shown in FIG. 21, a ground clearance 2146 between groundplane 2022 of routing layer 2010 and signal via 610 corresponds to theantipad 2030 shown in FIG. 20 and described above. The ground clearance2146 can be small in comparison with ground clearance 2144 and also canbe small in comparison with a ground clearance 2148 in routing layersabove the breakout layer 2112, provided that the ground plane 2022 doesnot contact signal via 610. As shown in FIG. 21, the signal trace 2140is in close proximity to the ground plane 2022 in a region 2150 due tothe relatively small diameter of antipad 2030.

Without being bound by any particular theory of operation, the inventorsbelieve that the portions of ground plane 2022 remaining in area 2028between signal vias 610 and 612 acts as a bridge that has the effect ofshorting opposing walls of the cavity surrounding the signal vias 610and 612. As can be seen in FIG. 20, that cavity is effectivelyrectangular and the bridge connects the long sides of the cavity. Thelowest resonant frequency that can be supported by a cavity is inverselyproportional to the length of the longest side of the cavity. Shortingthe opposing long sides effectively halves the length of those sides,doubling the lowest resonant frequency that can be supported in thecavity at that location.

In addition, the excitation of certain other undesirable yet higherfrequency resonant modes possible in the cavity may also beadvantageously shorted out due to the fact that ground plane 2022 can beconfigured to provide a more or less complete conductive wall across theentire opening of the cavity with only two relatively small openingscompared to the overall cross-sectional dimensions of the cavity. In thecase where it is possible to fabricate signal vias 610 and 612 as blindvias, for example by using sequential lamination to construct the board,ground plane 2022 below the signal trace breakout could beadvantageously configured with no antipad openings at all.

The additional conductive ground plane material can be thought of asdoing two things that help control undesired resonances: 1) shorting outthe cavity that supports the resonance, and 2) providing a path forcounter-to-signal-running ground currents in the trace-to-via transitionregion, that can serve to cancel some of the electromagnetic fieldsradiated by the two traces of a signal pair, both when they either arerouted coupled or separated, and both when they either are drivendifferentially with respect to each other or driven in common-modefashion with respect to ground.

This bridge is adjacent the breakout layer where a signal transitionfrom the signal vias to traces in the printed circuit board might occur.Mismatch in the coupling can excite resonances in the cavity. Bridgingthe long sides adjacent the breakout layer effectively doubles thelowest frequency at which the cavity will resonate. Because exciting aresonant mode in the operating frequency of the interconnection systemcan result in significant performance degradation, doubling the lowestfrequency at which a resonant mode might occur can substantiallyincrease the operating frequency range of the interconnection system.Accordingly, providing bridging adjacent to the breakout layer, wheresuch resonant modes might be excited, is thought to greatly increase theoperating frequency range of the interconnection system.

In the embodiment illustrated, the bridging is in a ground plane belowthe breakout layer. In some embodiments, such bridging material may beone or, in some embodiments, two or more layers below the routing layerimmediately adjacent the breakout layer. In other embodiments, thebridging may be one or two or more layers above the breakout layer.Alternatively or additionally, in some embodiments, the bridging may beon the breakout layer. Such a configuration may be implemented, forexample, with a ground structure patterned from the same conductivelayer as the signal traces. That ground structure, rather thanconnecting to a signal via, may be connected to a ground via.

An example of such a breakout layer is illustrated in FIG. 23. FIG. 23illustrates a region 2310 below an antipad, which is rectangular in theillustrated embodiment, in other layers. The conductive structuresillustrated in FIG. 23 may be patterned in the same conductive layerusing conventional printed circuit board fabrication techniques. Whenformed into a printed circuit board, pads on the ends of traces 2330 and2332 are pierced by signal vias (not shown).

A bridging region 2320 is pierced by ground vias 2340 and 2342. Groundvias 2340 and 2342 may be shadow vias or any other suitable conductivestructures that connect bridging region 2320 to ground. A groundedbridging region may provide a conductive bridge between sides of thecavity, reducing resonances excited in the breakout layer in thetransition of signals between traces 2330 and 2332 and the signal vias(not shown). In addition or in place of ground vias 2340 and 2342 ofFIG. 23 placed as shown, it can be advantageous to have other groundvias of whatever size as is convenient in other locations around theperiphery of the shaded region shown, especially with regard to thesuppression of various other undesired resonant modes in the cavity.

In the embodiment illustrated in FIG. 21, a printed circuit board hasantipads on ground planes in routing layers below the breakout layerthat are the same. It is not a requirement that the ground planes, orthe antipads around the signal pair, be the same on all routing layers.The antipads on ground planes below the breakout layer may be the sameas shown in ground plane 2022 or may have any other suitableconfiguration.

An embodiment of a breakout layer is shown in FIG. 22. In particular,FIG. 22 shows a breakout layer 2210 having a first conductive pad 2220connected to a signal via (not shown) and a second conductive pad 2222connected to a second signal via (not shown). A first signal trace 2230is connected to first conductive pad 2220 and a second signal trace 2232is connected to second conductive pad 2222. The signal traces 2230 and2232 may form a differential signal pair for connection of the signalvias to electrical circuitry or to other signal vias.

Depending on the routing of signal traces 2230 and 2232 betweenrespective conductive pads 2220 and 2222 and their final destinations inbreakout layer 2210, the signal traces may have different lengths. Inthe embodiment of FIG. 22, signal traces 2230 and 2232 bend to theright, such that signal trace 2230 would be longer than signal trace2232 in the absence of compensation. The different lengths of the signaltraces 2230 and 2232 may result in signal skew, which may produceundesired effects in the operation of the associated electricalcircuitry.

It might be expected that signal traces 2230 and 2232 would be routed tothe respective conductive pads 2220 and 2222 in a symmetrical manner toavoid such skew. However, the differences in the signal traces 2230 and2232 in other portions of the routing layer 2210 may produce signal skewwhich can be compensated by routing the signal traces 2230 and 2232 torespective pads 2220 and 2222 in a non-symmetrical manner. As shown inFIG. 22, signal trace 2232 is routed so as to have a compensatingsegment 2240 that is not present in signal trace 2230. In the embodimentof FIG. 22, the compensating segment 2240 adds to the length of signaltrace 2232 as compared with signal trace 2230. In other embodiments, thecompensating segment is wider or narrower than the signal traces tocompensate for signal skew. Thus, the width and/or the length of thecompensating segment may be adjusted to compensate for signal skew. Thecompensating segment 2240 may compensate for the differences in thelengths, widths and/or other characteristics of signal traces 2230 and2232 along their signal paths. It will be understood that in othersituations, a compensating segment may be added to signal trace 2230rather than to signal trace 2232.

The offset routing of the signal traces 2230 and 2232 takes advantage ofthe added ground plane conductive material in the region between the twosignal vias of the differential pair to allow for a longer path of oneof the two signal traces electrically referenced to that ground planecompared with the length of the path of the other signal trace, in orderto allow for compensation of mismatch in length or delay caused by, forexample, a right angle bend of the differential signal traces as theyare routed into the via pattern and connected to the signal vias. Thecompensating segment 2240 may be located adjacent to one or more groundplanes, such as ground plane 2022 in which two small (e.g. circular)antipads 2030 are formed around the signal vias, as shown in FIGS. 20and 21. If there were no adjacent ground plane in the region between thesignal vias, then the compensating segment 2240 would have a higherimpedance to ground, resulting in an imbalanced pair.

The right angle bend is one way of keeping the two traces of the paircoupled uniformly to each other as long as possible, before one of themhas an additional length added right before it joins the signal via.This adjustment of relative lengths of the two paired traces isperformed in a region where at least one adjacent ground plane orcoplanar-to-signal-traces partial ground plane can serve as a conductorof ground return currents for each of the paired traces, especiallywhere these traces diverge and run separately from each other for somedistance. In order to spread apart to join the vias that are typicallyspaced at least, say 1.0 mm, apart, whereas the paired traces might betypically spaced at most, say 0.3 mm apart, the two paired traces willbecome less electrically coupled from each other, reducing the fieldcancellation benefits of differential signaling, which leads to anincrease in radiated fields from this transition that excite undesirableresonant modes in the cavity. However, this radiation can be reduced byconfiguring appropriate adjacent ground conductive paths, which supportground currents that generally run counter to the signal currentdirection and act to cancel a portion of the radiated fields.

Further embodiments of a printed circuit board are described withreference to FIGS. 24-27. A partial top view of an embodiment of anattachment layer, such as attachment layer 560 (FIG. 5), of thebackplane 110 is shown in FIG. 24. In the case of multiple attachmentlayers, each of the attachment layers of backplane 110 may have the sameconfiguration. FIG. 24 shows a via pattern 2400 of a connector footprint2402 of backplane connector 100. A similar via pattern may be used on adaughter card, but is not expressly shown.

It should be appreciated that FIG. 24 is partially schematic in that allof the illustrated structures may not in all embodiments be seen in avisual inspection of the top of a printed circuit board. A coating maybe placed over the board that obscures some of the structure. Inaddition, some structures may be formed on layers below the surface ofthe board. Those layers may nonetheless show in a top view so that therelative position of structures in the layers may be understood. Forexample, signal traces and ground planes may not both be visible in thesame view of the board, as they are on different vertical planes withinthe printed circuit board. However, because the relative positioning ofsignal and ground structures may be important to performance of aprinted circuit board, both may be shown in what is referred to as a topview.

In the example illustrated, via pattern 2400 of attachment layer 560includes a first signal via 2410 and a second signal via 2412, whichform a differential signal pair. The signal vias 2410 and 2412 extendvertically through the attachment layers and have diameters andlocations in attachment layer 560 that are selected to accept thecontact tails 140 of backplane connector 100. In forming the board, aground plane 2420 is partially removed, such as by patterning a copperlayer on a laminate, to form a non-conductive area, or antipad 2422,forming a ground clearance, surrounding signal vias 2410 and 2412, sothat the dielectric sheet of attachment layer 560 is exposed. The areaswhere the ground plane is removed may be called “non-conductive areas”or “antipads.” The antipad 2422 has a size and shape to precludeshorting of ground plane 2420 to signal vias 2410 and 2412, even ifthere is some imprecision in forming the vias relative to ground plane2420, and to establish a desired impedance of the signal path formed bysignal vias 2410 and 2412. The ground plane 2420 is removed aroundsignal vias 2410 and 2412 and, when the signal vias form a differentialsignal pair, may be removed between signal vias 2410 and 2412 in some orall of the layers. In the embodiment of FIG. 24, antipad 2422 isrectangular in shape. However, it should be appreciated that antipad2422 may have any suitable shape, including elliptical, and may haverounded corners.

Via pattern 2400 of attachment layer 560 further includes ground vias2430, 2432, 2434 and 2436 associated with signal vias 2410 and 2412. Theground vias may be disposed around the signal vias. In this example,ground vias 2430 and 2432 may be located adjacent to one end of the viapattern 2400 and to signal via 2410, and ground vias 2434 and 2436 maybe located adjacent to an opposite end of the via pattern 2400 and tosignal via 2412. The ground vias 2430, 2432, 2434 and 2436 may belocated more or less in proximity to the respective corners ofrectangular antipad 2422. The ground vias 2430, 2432, 2434 and 2436 aredimensioned and located to accept corresponding contact tails 140 ofbackplane connector 100. The ground vias interconnect the ground planesof some or all of the layers of the backplane 110. In particular, theground vias may extend through all of the layers of the backplane 110and may be plated with a conductive material.

Each of the ground vias 2430, 2432, 2434 and 2436 may extend through theattachment layers and through the routing layers of backplane 110. Insome embodiments, each ground via is formed as a through hole thatextends through the entire thickness of backplane 110. Each ground viamay have a uniform diameter throughout its length, but is not requiredto have a uniform diameter, provided that the diameter in the attachmentlayers of backplane 110 is sufficient to accept a contact tail ofbackplane connector 100. Each ground via includes contact pads on someor all of the attachment layers and the routing layers. In someembodiments, each ground via passes through multiple ground planes onthe attachment layers and the routing layers.

A ground via may have any suitable diameter, such as between 12 mils and25 mils. In one non-limiting example, each ground via has a diameter of15.7 mils and includes contact pads having diameters of 25.7 mils onevery ground layer. It will be understood that these dimensions are notlimiting and that other dimensions may be utilized.

Via pattern 2400 of attachment layer 560 further includes ground shadowvias 2440, 2442, 2444 and 2446 located adjacent to signal vias 2410 and2412. The ground shadow vias 2440 and 2442 may be located on oppositesides of signal via 2410, and shadow vias 2444 and 2446 may be locatedon opposite sides of signal via 2412. In the example of FIG. 24, groundshadow vias 2440 and 2442 are located along a line 2450 that passesthrough signal via 2410 and ground shadow vias 2444 and 2446 are locatedalong a line 2452 that passes through signal via 2412, where lines 2450and 2452 are perpendicular to a line 2454 that passes through signalvias 2410 and 2412. However, it will be understood that the groundshadow vias 2440, 2442, 2444 and 2446 may have other locations adjacentto signal vias 2410 and 2412. As shown in FIG. 24, the ground shadowvias overlap the edges of antipad 2422 and are positioned relativelyclosely to the respective signal vias. The ground shadow vias do notaccept contact tails of backplane connector 100. The ground shadow vias2440, 2442, 2444 and 2446 are discussed in greater detail below.

A partial top view of an embodiment of a routing layer, such as routinglayer 570 (FIG. 5), of backplane 110 is shown in FIG. 25. In the case ofmultiple routing layers, each of the routing layers may have the sameconfiguration, except that different electrical connections are made tothe signal vias in different via patterns. In some embodiments, some ofthe signal vias and/or ground vias may be backdrilled, removingconductive plating close to a lower surface of the printed circuitboard. A via pattern 2500 is shown in FIG. 25. It will be understoodthat via pattern 2500 is vertically aligned under respective via pattern2400 of the attachment layers. It will further be understood that thevia pattern 2500 is not visible in the backplane 110 after the layers ofthe backplane are pressed together, such that FIG. 25 may be regarded asa schematic illustration of the routing layer.

Via pattern 2500 of routing layer 570 includes signal vias 2410 and 2412which extend vertically through the attachment layers and at least oneof the routing layers. In the embodiment of FIGS. 24 and 25, the signalvias 2410 and 2412 have smaller diameters in the routing layers than inthe attachment layers. In particular, signal vias 2410 and 2412 have afirst diameter in the attachment layers and a second diameter in therouting layers, wherein the second diameter is smaller than the firstdiameter. The signal vias 2410 and 2412 can have smaller diameters inthe routing layers because they are not required to accept the contacttails 140 of the backplane connector 100, and the smaller diameters inthe routing layers provide increased area for routing of signal tracesto the signal vias. In the via pattern 2500, a ground plane 2530 ispartially removed to form an antipad 2532 surrounding signal vias 2410and 2412. The antipad 2532 of routing layer 570 may have the same sizeand shape as the antipad 2422 of attachment layer 560. However, this isnot a requirement and, in some embodiments, the separation between thesignal vias and the edges of the ground plane may be selected at eachlayer to provide a desired impedance or to otherwise provide desiredelectrical properties.

Via pattern 2500 of routing layer 570 also includes ground vias 2430,2432, 2434 and 2436 which have the same locations and configurations asthe corresponding ground vias in attachment layer 560. In particular,the ground vias 2430 and 2432 are located adjacent to one end of the viapattern 2500 and adjacent to signal via 2410, and ground vias 2434 and2436 are located adjacent to an opposite end of the via pattern 2500 andadjacent to signal via 2412. The ground vias in the routing layers arenot required to accept contact tails of the backplane connector, but mayhave the same diameters as the ground vias in the attachment layers. Theground vias 2430, 2432, 2434 and 2436 in the routing layers can beplated or filled with a conductive material. As noted, the ground viastypically interconnect the ground planes of all the layers of thebackplane 110.

Via pattern 2500 of routing layer 570 further includes ground shadowvias 2440, 2442, 2444 and 2446 which extend vertically from theattachment layers. In both the attachment layers and the routing layers,the ground shadow vias are plated or filled with a conductive material.In the embodiment of FIGS. 24 and 25, the ground shadow vias 2440, 2442,2444 and 2446 have larger diameters in the routing layers than in theattachment layers. In particular, ground shadow vias 2440, 2442, 2444and 2446 have a first diameter in the attachment layers and a seconddiameter in the routing layers, wherein the second diameter is largerthan the first diameter. As discussed below, the ground shadow vias areconfigured relative to the signal vias to provide impedance matching andshielding between adjacent differential signal pairs.

A schematic cross-section of the backplane 110 of FIGS. 24 and 25 isshown in FIG. 26. FIG. 27 is a perspective view of the via pattern ofFIGS. 24 and 25, with the layers of backplane 110 omitted for purposesof illustration. The spatial relationship of the signal vias 2410 and2412, and the ground shadow vias 2440, 2442, 2444 and 2446 is shown inFIG. 27.

The interrelationship between the vias of via patterns 2400 and 2500 atdifferent depths in backplane 110 is shown in FIG. 26. As shown, signalvia 2410 extends through attachment layers 560, 562, etc. and one ormore of routing layers 570, 572, etc., and provides a signal connectionto at least one of the routing layers, known as a “breakout layer”. Abreakout layer 2674 is shown in FIG. 26, with a contact pad 2680 onsignal via 2410 connecting to a signal trace (not shown) on breakoutlayer 2674. Ground shadow vias 2440 and 2442 are located adjacent tosignal via 2410 and are plated or filled with a conductive material inboth the attachment layers and the routing layers. Ground shadow vias2444 and 2446 are located adjacent to signal via 2412 and have a similarconfiguration. The ground shadow vias interconnect ground planes of therouting layers and the attachment layers. The ground vias 2430, 2432,2434 and 2436 are not shown in the cross-section of FIG. 26.

As shown in FIG. 26, the signal via 2410 extends through the attachmentlayers and through at least one of the routing layers of backplane 110.Such a via may be formed by drilling a hole fully through the board,plating the via and then removing portions of the plating adjacent thelower surface of the board.

In the embodiment of FIG. 26, signal via 2410 has a first section 2600having a first diameter 2610 and a first length 2612 and a secondsection 2614 having a second diameter 2620. In general, the firstsection 2600 extends through the attachment layers and the secondsection 2614 extends through at least one of the routing layers. Thefirst diameter 2610 is larger than the second diameter 2620. Asindicated previously, the first diameter 2610 is selected to accept acontact tail 140 of backplane connector 100. The signal via 2410 isplated with a conductive material. The signal via 2410 may have acontact pad 2630 on the top layer of the backplane 110 and contact pad2680 on the breakout layer 2674 where the signal trace is connected.

As further shown in FIG. 26, ground shadow via 2440 extends through theattachment layers and through the routing layers of backplane 110. Theground shadow via 2440 includes a first section 2650 having a firstdiameter 2652 and a length 2612, and a second section 2660 having asecond diameter 2662 and a second length 2664. The ground shadow via2440 may be formed as a through hole that extends through the thicknessof backplane 110. However, shadow via 2440 is not limited to a throughhole. The length 2612 of the first section 2650 of ground shadow via2440 may be somewhat less than, equal to or somewhat greater than thelength of the first section 2600 of signal via 2410 which accepts thecontact tail of backplane connector 100. Ground shadow via 2440 isprovided with contact pads (not shown) which may connect to some or allground planes in the attachment layers and the routing layers. Theground shadow via 2440 is not required to accept a contact tail ofbackplane connector 100.

In one non-limiting example, the signal vias 2410 and 2412 havedimensions where the first diameter 2610 of the first section 2600 is 14to 16 mils and the second diameter 2620 of the second section 2614 is 10to 12 mils. The first length 2612 of the first section 2600 may be 50 to60 mils. The ground shadow vias may be reversed in dimensions. Inparticular, the first diameter 2652 of the first section 2650 is 10 to12 mils and the second diameter 2662 of the second section 2660 is 14 to16 mils. In some embodiments, the ground shadow vias have diameters of 6mils. It will be understood that these dimensions are not limiting andthat other dimensions may be utilized.

The signal via 2410 may be formed by drilling a through hole having thesmaller second diameter 2620 entirely through backplane 110 and thendrilling the first section 2600 having the larger first diameter 2610from the front surface to a desired depth. The dual diameter hole isthen plated in upper first section 2600 and is plated or filled withconductive material in lower second section 2614. The plating in theportion of signal via 2410 below the breakout layer 2674 may be drilledout to avoid undesired signal effects.

The ground shadow via 2440 may be formed by drilling a through holehaving the smaller first diameter 2652 entirely through backplane 110and then drilling the second section 2660 having the larger seconddiameter 2662 from the back surface to a desired depth. The groundshadow via 2440 may then be plated or filled with a conductive materialthrough its entire length.

As noted above, the dual diameter signal via 2410 has larger firstdiameter 2610 selected to accept the contact tails of the matingconnector. The lower second section 2614 of the signal via 2410 hassmaller second diameter 2620 to provide additional space for routing ofsignal traces. With this arrangement, it is difficult to achieveimpedance matching with current connector densities while maintaininghigh levels of isolation between signal vias. The dual diameter groundshadow vias 2440, 2442, 2444 and 2446 described herein provide shieldingbetween adjacent differential signal pairs as well as impedancematching. In particular, the dual diameters of the signal vias and theground shadow vias have a reversed diameter configuration in theattachment layers and the routing layers to provide impedance matching.In some embodiments, the larger first diameter 2610 of signal via 2410is equal to the larger second diameter 2662 of ground shadow via 2440,and the smaller second diameter 2620 of signal via 2410 is equal to thesmaller first diameter 2652 of ground shadow via 2440 to provideimpedance matching. However, it should be appreciated that thedimensions need not be exact. In accordance with some embodiments, forexample, the larger first diameter 2610 of signal via 2410 may be within+/−20% of the larger second diameter 2662 of ground shadow via 2440.Similarly, the smaller second diameter 2620 of signal via 2410 may bewithin +/−20% of the smaller first diameter 2652 of ground shadow via2440. Further, the transitions between diameters of the signal tracesand the ground shadow vias are not necessarily abrupt and do notnecessarily occur at the same level in the layers of the backplane. Inparticular, the transitions between diameters of the signal vias 2410and 2412 may occur at a different level in the backplane 110 than thetransitions between diameters of the ground shadow vias 2440, 2442, 2444and 2446 in order to avoid close spacing or contact between the signalvias and the ground shadow vias at the transitions (see FIG. 26).

A number of features of printed circuit boards are shown and describedherein. It will be understood that the features may be utilizedseparately or in combination in a particular application, withoutdeparting from the scope of the present disclosure.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. For example, layers may bedescribed as upper layers, or “above” or “below” other layers. It shouldbe appreciated these terms are for ease of illustration and not alimitation on the orientation of layers. In the embodiment illustrated,“upper” refers to a direction towards a surface of a printed circuitboard to which components are attached. In some embodiments, componentsmay be attached to two sides of a printed circuit board, such that upperand lower may depend on which vias are being considered. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and the scopeof the present invention. Accordingly, the foregoing description is byway of example only and is not intended to be limiting. The presentinvention is limited only as defined in the following claims and theequivalents thereto.

What is claimed is:
 1. A printed circuit board comprising: a pluralityof layers including attachment layers and routing layers; and viapatterns formed in one or more of the plurality of layers, each of thevia patterns comprising: first and second signal vias extending throughthe attachment layers and connecting to respective signal traces on atleast one of the routing layers; ground vias extending through at leastthe attachment layers; and non-plated air holes associated with each ofthe first and second signal vias and extending to a predetermined depthin the plurality of layers, wherein each of the non-plated air holes islocated on a first line that passes through one of the first and secondsignal vias, the first line being perpendicular to a second line thatpasses through the first and second signal vias.
 2. The printed circuitboard as defined in claim 1, wherein the non-plated air holes arelocated on opposite sides of the first and second signal vias.
 3. Theprinted circuit board as defined in claim 2, wherein additionalnon-plated air holes are located on the second line through the firstand second signal vias.
 4. The printed circuit board as defined in claim1, wherein the non-plated air holes have smaller diameters than thefirst and second signal vias.
 5. The printed circuit board as defined inclaim 1, wherein the non-plated air holes extend through the attachmentlayers of the plurality of layers.
 6. The printed circuit board asdefined in claim 1, wherein one or more of the attachment layersincludes a ground plane and wherein the ground plane is removed in anarea around the first and second signal vias to form an antipad.
 7. Aprinted circuit board comprising: a plurality of layers includingattachment layers and routing layers; and via patterns formed in one ormore of the plurality of layers, each of the via patterns comprising:first and second signal vias extending through the attachment layers andconnecting to respective signal traces on one or more of the routinglayers; ground vias extending through at least the attachment layers;and shadow vias extending at least through the attachment layers, theshadow vias having smaller diameters than the first and second signalvias, wherein each of the shadow vias is located on a first line thatpasses through one of the first and second signal vias, the first linebeing perpendicular to a second line that passes through the first andsecond signal vias and wherein the shadow vias are located on oppositesides of the signal vias.
 8. The printed circuit board as defined inclaim 7, wherein the shadow vias are free of conductive material atleast in the attachment layers.
 9. The printed circuit board as definedin claim 7, wherein the shadow vias are plated or filled with aconductive material.
 10. The printed circuit board as defined in claim7, wherein each of the shadow vias extends to a predetermined depth inthe plurality of layers.
 11. The printed circuit board as defined inclaim 9, wherein the shadow vias extend through the attachment layersand the routing layers.
 12. A printed circuit board comprising: aplurality of layers including conductive layers separated by dielectriclayers; first and second signal vias forming a differential signal pair,the first and second signal vias extending through one or more of theplurality of layers; and non-plated air holes associated with each ofthe first and second signal vias and extending to a predetermined depthin the plurality of layers, wherein each of the non-plated air holes islocated on a first line that passes through one of the first and secondsignal vias, the first line being perpendicular to a second line thatpasses through the first and second signal vias.
 13. The printed circuitboard as defined in claim 12, wherein the non-plated air holes arelocated on opposite sides of the first and second signal vias.
 14. Theprinted circuit board as defined in claim 12, wherein the non-plated airholes have smaller diameters than the first and second signal vias. 15.The printed circuit board as defined in claim 12, further comprisingground vias extending through one or more of the plurality of layers.16. The printed circuit board as defined in claim 12, wherein at leastone of the conductive layers of the plurality of layers is removed in anarea around the first and second signal vias to form an antipad.
 17. Theprinted circuit board as defined in claim 16, wherein the non-plated airholes are located, at least partially, within the antipads.
 18. Theprinted circuit board as defined in claim 12, wherein the non-plated airholes are located adjacent to and on opposite sides of each of the firstand second signal vias.